1. Technical Field
This invention relates to display backlights and fluorescent lamps used in backlights.
2. Related Art
Cathode ray tubes (CRTs) are employed as display devices for many products. However, because of their size, weight, and power requirements, CRTs are generally unsuitable for small and light electronic products such as Personal Data Assistants (PDA), portable game machines, and other small products. Alternative displays have therefore been developed, such as liquid crystal display devices (LCDs) and plasma display panels (PDPs). LCDs find widespread use as flat display panels in laptop computers, desktop computers, televisions, and other devices because of their high quality image, lightness, thinness, compact size, and low power consumption.
LCDs display information by controlling transmittance of externally generated light through a liquid crystal layer. In some LCDs a backlight is placed behind the LCD panel to illuminate it. The backlight may be a direct illumination backlight or an edge illumination backlight.
In an edge illumination backlight, a lamp unit may be provided at a lateral side of a light-guiding plate. The lamp unit may include a fluorescent lamp, a protective lamp holder that receives one or both ends of the fluorescent lamp, and a reflective sheet that directs light emitted from the fluorescent lamp to the light-guiding plate. The edge illumination backlight may have a small enough profile to be used in small devices such as laptop computer displays to provide a uniform and long lighting life cycle.
Large LCDs, such as those used in televisions or large computer displays, may use a direct illumination backlight. In the direct illumination backlight, multiple fluorescent lamps may be arranged along a lower surface of a light-diffusion sheet to directly illuminate the LCD. The direct illumination backlight is more efficient than the edge illumination backlight and may provide more luminance.
In FIG. 1, the backlight 100 may include multiple fluorescent lamps 1, an outer case 3, and light-scattering layers 5a, 5b and 5c. The fluorescent lamps 1 may be arranged at intervals in the case 3. The case 3 may fix or support the fluorescent lamps 1 on one or both sides of the case 3.
The light-scattering layers 5a, 5b and 5c may be provided between the fluorescent lamps 1 and an LCD panel (not shown). The light-scattering layers 5a, 5b and 5c may reduce or eliminate the reflection of silhouettes of the fluorescent lamps 1 on the LCD panel, and also may enhance uniform luminance. The layers 5a, 5b, and 5c may include multiple light diffusion sheets and a diffusion plate between the fluorescent lamps 1 and the LCD panel. A reflective sheet 7 may be present inside the outer case 3 and may concentrate light emitted from the fluorescent lamps 1 on the LCD panel.
In FIG. 2, each fluorescent lamp 1 may include internal electrodes 2a and 2b for receiving power. Each fluorescent lamp 1 may be a Cold Cathode Fluorescent Lamp (CCFL) filled with a discharge gas. Power supply wires 9a and 9b are connected to the electrodes 2a and 2b and to a driving circuit through the connector 11. The power supply wire 9b is connected to the electrode 2b and the connector 11 and the power supply wire 9a is connected to the electrode 2a and the connector 11. One or both of the power supply wires 9b and 9a may run under or around a side of the outer case 3 to meet with the connector 11 when the fluorescent lamps are positioned in the case 3.
For each of the multiple fluorescent lamps 1, power supply wires couple the lamp electrodes to a separate connector. The number of wires may be large and their arrangement or routing around the outer case 3 may become complicated, decreasing the operating efficiency, increasing the fabrication cost and complexity, and lowering the yield. In addition, the fluorescent lamp electrodes extend through holes in the outer case 3 to provide access for the power supply wires. In this configuration, the changing of a lamp and routine maintenance on the backlight 100 may be difficult.
FIG. 3 shows an External Electrode Fluorescent Lamp (EEFL) 300 developed as an alternative to the fluorescent lamps 1 with internal electrodes. The EEFL 300 may include an external positive (+) electrode 33a and an external negative (−) electrode 33b at either end of a tube 31. Compared to Internal Electrode Fluorescent Lamps (IEFL) such as that shown in FIG. 2, the EEFL 300 may have a longer lifespan and may run from a single inverter, resulting in size and weight advantages.
Luminance may vary over the length of the EEFL 300. The end of the tube 31 with the positive (+) electrode 33a may have a relatively high luminance, while the end of the tube 31 with the negative (−) electrode 33b may have a relatively low luminance. The single inverter also may result in a slow start-up speed. Accordingly, the EEFL 300 is not always suitable for a large display such as a television or computer monitor.
FIG. 4 shows a perspective view of a direct illumination backlight 400 that includes EIFLs. The External Internal Fluorescent Lamp (EIFL) may be an alternative to the CCFL and EEFL in a direct illumination backlight. FIG. 5 shows an EIFL that may be used in the backlight 400.
In FIG. 5, the EIFL 44 may include a fluorescent substance (not shown) coated on an inner surface of a glass tube 42. The EIFL 44 may also include discharge gas such as an inert rare gas mixed with hydrargyrum injected into the glass tube 42. The EIFL 44 may also include an external electrode 56 and an internal electrode 58 that extends into the glass tube 42. The external electrode 56 may provide the positive (+) electrode on one end of the glass tube, and the internal electrode 58 may provide the negative (−) electrode at the opposite end of the glass tube 42. A component 45 on the outside of the glass tube 42 may secure or support the internal electrode 58.
In FIG. 4, the backlight 400 may include multiple EIFLs 44, supports 46 and 48, and power connection plates 50. The backlight 400 may also include a reflective sheet 52 and a light-diffusion sheet 54. The EIFLs 44 may be arranged across the backlight 400. The positive electrodes 58 of the EIFLs 44 may couple to the support 48 and the negative electrodes 56 may couple to the support 46.
Power may be applied to the EIFLs 44 through the supports 46 and 48 and through the power connection plates 50 underneath the supports 46 and 48. The reflective sheet 52 may be provided below the EIFLs 44 to reflect the light emitted from the EIFLs 44 to the LCD panel. The light-diffusion sheet 54 may be provided above the EIFLs 44 to diffuse the light reflected by the reflective sheet 52.
One or both support members 46 and 48 may comprise conductive silicon rubber that supports the EIFLs 44. The support 46 may include multiple insertion holes 53a that may accept the external electrodes 56. A conductor 51 may couple one or more external electrodes 56 to apply a positive (+) voltage to the external electrodes 56. The support 48 may include multiple insertion holes 53b that may accept the components 45 that support the internal electrodes 58. A conductor 55 may couple one or more of the internal electrodes 58 to apply the negative (−) voltage to the ElFLs 44.
The power connection plate 50 may include a copper material such as a copper sheet or a copper tape. The reflective sheet 52 may include concave or convex portions that may alternate along a longitudinal direction and may reflect the light emitted from the fluorescent lamp 44 to the LCD panel. The concave portions of the reflective sheet 52 may accept the EIFLs 44. The light-diffusion sheet 54 may be spaced from the fluorescent lamps 44 to prevent the EIFL silhouettes from being cast to the LCD panel.
In FIG. 6, an inverter that drives the backlight 400 may include a high voltage generator 60. The generator 60 produces the driving voltage for the parallel connected EIFLs 44. The driving voltage may be applied to the external electrodes 56 through the conductor 51, causing current flow to the internal electrodes 58 and through the lead wire 55 to generate electric discharge in the EIFLs 44. The external electrodes 56 may serve as the positive electrodes and the internal electrodes 58 may be grounded. The inverter may drive multiple EIFLs simultaneously.
As the size of the display increases, the length of the EIFLs 44 increases. One consequence is that the driving voltage that starts electric discharge in the EIFLs 44 increases. The driving voltage that the inverter is able to produce may limit the size of the display.
FIG. 7 shows additional details of an EIFL 70. The EIFL 70 may include an external electrode 72 outside and at one end of a tube 71, and an internal electrode 73 provided inside and at the other end of the tube 71. The EIFL 70 also includes an internal electrode glass bead 74 that may support or secure the power conductor 76, and includes an external electrode glass bead 78. The tube 71 may be coated with a fluorescent substance 75. The power conductor 76 may couple the internal electrode 73.
The EIFL fabrication process separately forms and fixes the glass beads 74 and 78 at opposite ends of the tube 71. Fabricating the glass beads 74 and 78 may be a complex and costly process. The high driving voltage applied to the external electrode 72 also may result in particularly strong electric fields between the external electrode glass bead 78 and the tube 71. The resulting stress on the EIFL 70 may reduce its reliability and operating life.
The invention is directed to a fluorescent lamp and backlight that overcomes one or more of the potential drawbacks in the related art.